Stephen Gire and other health researchers on the ground in Africa had some hope that the Ebola outbreak was coming under control or at least plateauing in late May. Then came the funeral of a healer in Guinea. More than a dozen of the mourners contracted the disease there, probably by washing or touching the body, and took it to Sierra Leone, according to a new DNA mapping of the Ebola virus that scientists hope will help them understand what makes this killer tick.

''You had this huge burst after it looked like the outbreak was starting to die down,'' Gire said. ''It sort of threw a wrench in the response.''

Ebola exploded after that funeral and has now killed at least 1,552 people in West Africa. It's probably more than that, with 40 percent of the cases in the last three weeks, according to the World Health Organization. WHO officials said Thursday the outbreak continues to accelerate and could reach more than 20,000 cases eventually.

Gire and more than 50 colleagues - five of whom died from Ebola while fighting the outbreak in Africa - have mapped the genetic code of this strain of Ebola, and in so doing showed how crucial that May funeral was. They hope to use that to track mutations that could become more worrisome the longer the outbreak lasts. This detailed genetic mapping also could eventually make a bit of a difference in the way doctors spot and fight the disease, especially with work in preliminary vaccines.

On Thursday, officials at the National Institutes of Health announced that they were launching safety trials on a preliminary vaccine for Ebola. Researchers have already checked that still-not-tested vaccine against some of the more than 350 mutations in this strain of Ebola to make sure the changes the disease is making won't undercut science's hurried efforts to fight it, said Pardis Sabeti, a scientist at Harvard University and its affiliated Broad Institute.

She and Gire, also at Broad and Harvard, are two of the lead authors of a study, published Thursday in the journal Science, that maps the killer disease strain based on specimens collected from 78 patients.

The virus has mutated more than 300 times from previous strains of Ebola, Gire said. Researchers have also pinpointed about 50 places in the genetic code where the virus has changed since this outbreak